Deaths from the hospital superbug MRSA could double over the next five years, experts have warned.
All types of MRSA appear to be developing resistance to vancomycin
UK scientists say the methicillin-resistant Staphylococcus aureus bacteria is becoming increasingly resistant to the antibiotic vancomycin.
Around 5,000 Britons die each year from infections acquired in hospital.
But the researchers, whose study is published in Emerging Infectious Diseases, say more lives will be lost if vancomycin can no longer be used.
It had been thought that vancomycin-resistance was only a problem in one strain of MRSA - seen in the US and Japan.
But scientists from the University of Bath, University of Bristol and Southmead Hospital say all five major types of MRSA - including those seen in the UK - show signs of resistance to vancomycin.
The study was carried out in the UK, USA, France, Japan, Sweden, Poland, Norway and China.
Around 7,000 patients are currently affected by MRSA each year in the UK each year.
The researchers warn vancomycin-resistance is likely to become more common as more of the antibiotic was used to treat growing numbers of MRSA
This would mean that doctors would have to try "cocktails" of antibiotics to find the right combination to treat an individual's infection - something which could take days, increasing the risk to the patient.
Three cases of MRSA bacteria which are totally resistant to vancomycin have been reported in the US, but fully-resistant bacteria have not yet been found in the UK.
The type of bacteria with increased resistance is known as VISA - vancomycin-intermediate Staphylococcus aureus.
The scientists believed that it may be an intermediate stage to the development of bacteria which are fully resistant to vancomycin.
Dr Mark Enright of the University of Bath who led the research, told BBC News Online: "With increasing vancomycin-resistance, we are going to see a significant increase in mortality.
"If we lose vancomycin completely as a treatment, we could see a doubling in deaths over the next five years."
He said there were still treatments that would work.
Dr Enright, a senior research fellow in the Department of Biology and Biochemistry, added: "You could treat some types of infection with some antibiotics sometimes.
"But doctors are going to lose a few days working out how they can treat the patient."
He added: "The results of our study show that the problem is much more serious than was previously thought."
A spokesperson for the Health Protection Agency said "The number of blood stream infections caused by MRSA has increased in the UK in recent years.
"However, despite carrying out thousands of tests over the last six years, we haven't identified any MRSA in the UK that are fully resistant to vancomycin (only three such isolates are known to exist, all from the USA).
"Moreover, only a handful of UK samples have been found to show a low level of resistance to vancomycin.
"Infections caused by MRSA resistant to vancomycin (either fully or at low-level) are treatable with other antibiotics such as linezolid."